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Astron. Astrophys. 321, L25-L28 (1997)


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Letter to the Editor

On the nature of SMC X-1

X.-D. Li 1, 2 and E.P.J. van den Heuvel 1

1 Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 Department of Astronomy, Nanjing University, Nanjing 210093, P.R. China

Received 3 October 1996 / Accepted 7 January 1997

Abstract

The 0.71 s X-ray pulsar SMC X-1 has some distinct features from other X-ray pulsars. It maintained a stable spin-up though in X-rays both low- and high-intensity states have been observed. An X-ray burst was discovered from SMC X-1, and was probably generated by an instability in the accretion flow. Using the modified magnetically threaded accretion disk theory, we have estimated the magnetic moment of SMC X-1 to be [FORMULA] G cm3, which is lower than those of other typical X-ray pulsars (e.g., Her X-1, Vela X-1) by an order of magnitude. Comparing SMC X-1 with the new transient X-ray pulsar GRO J1744-28, from which type II bursts were recently discovered, we suggest that the nature of this type of "bursting pulsars" may be accounted for by their relatively low magnetic moments and high accretion rates, if the burst from SMC X-1 is really due to spasmodic accretion as those from GRO J1744-28. The inner edge of the accretion disk in both X-ray sources is found to lie in the transition region at which the radiation pressure becomes comparable to the gas pressure, suggesting that the bursts from both sources may be related to the Lightman-Eardley instability in the inner region of the disk. The difference between the one burst from SMC X-1 and the many bursts from GRO J1744-28 is discussed, and may originate from the different magnetic field structure in these two X-ray pulsars.

Key words: accretion, accretion disks – binaries: close – stars: neutron – pulsars: SMC X-1

Send offprint requests to: X.-D. Li

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998

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